The present invention relates to electroluminescent semiconductor devices, and in particular to an electroluminescent semiconductor device in which the active material is a body of gallium nitride. The invention also relates to a method for making such electroluminescent semiconductor devices.
Electroluminescent semiconductor devices of gallium nitride are known in the art. An example of such devices is shown and described in U.S. Pat. No. 3,683,240 granted to J. I. Pankove. The gallium nitride electroluminescent device described in this patent comprises a substrate of an electrically insulating material which is optically transparent, such as sapphire. On a surface of the substrate is a body of N type conductive crystalline gallium nitride, which has a conductivity of about 100 mhos, and on the surface of this N type conductivity body is a thin body of insulating crystalline gallium nitride. The conductive and insulative bodies are epitaxially deposited on the sapphire substrate by the vapor phase epitaxy technique. During the initial step of the deposition process little or no acceptor impurity is included so that the initial portion of the deposited gallium nitride is conductive to form the conductive body. When this conductive body has attained a desired thickness, a sufficient amount of acceptor impurities is included so as to compensate all of the native, uncontrolled donors, such as nitrogen vacancies, which are inherently formed in the material, thereby forming the insulating gallium nitride body. One electrical contact is provided on a surface of the insulating gallium nitride body and another contact is provided on the periphery of the conductive gallium nitride body. When a D.C. potential is applied between the two contacts, high electric fields are generated in the insulating gallium nitride body which cause the release of electrons trapped in the acceptor centers and a subsequent avalanche multiplication of free electrons and holes. Light in the blue to green region is emitted by the insulating gallium nitride body when these carriers are recombined and can be seen through the substrate.
Electroluminescent semiconductor devices of gallium nitride are often employed in conjunction with integrated circuits by sharing a common D.C. source which is typically 12 volts or less. The operating voltage of gallium nitride electroluminescent devices is found to be largely dependent on the physical properties of the insulating gallium nitride body, particularly, on the thickness of the insulating body. The insulating gallium nitride body having a thickness sufficiently small to meet the low voltage requirement of integrated circuits will cause an electrical breakdown in the body, which is detrimental to the electroluminescent device. Therefore, the insulating gallium nitride body is required to have a substantial thickness and therefore a high operating voltage if reliable performance is to be assured.
In more detail, the conductive gallium nitride body is currently considered to have a heterogeneous structure due to lattice imperfections caused by different crystallographic properties of the material between different depths and by nonuniform growth rates at which the conductive gallium nitride is deposited. Therefore, the insulative gallium nitride body that is deposited on such a conductive gallium nitride body tends to maintain the same lattice orientation as the underlying body. Furthermore, such imperfections are particularly pronounced at the interface between the two bodies due to the difference in lattice coefficient between them. When a D.C. current is passed through the insulative body, the lattice imperfections in that body become the centers of field concentration where avalanche multiplication of free electrons and holes takes place to such an extent that the resistance value of the material in that field centers is reduced. This in turn enhances the field concentration and a large current will result which breaks down the material. In applications where the thickness of the insulating gallium nitride body is held at a value less than 1 micrometer, lattice imperfections thus become a factor which cannot be ignored.